A cooling system for a motor vehicle may include one or more heat exchangers that draw heat from an engine exhaust flow. An exhaust-gas recirculation (EGR) cooler is one such heat exchanger. Liquid coolant in the heat exchanger may circulate in a closed loop that includes a radiator. From the radiator, excess heat is discharged to the ambient air. In some configurations and scenarios, the heat from the exhaust flow may greatly increase the temperature and vapor pressure of the coolant. The conduits of the cooling system must therefore maintain the coolant at an elevated pressure to avoid boiling.
In addition, some measures may be taken to limit the maximum temperature of the coolant, and thereby limit the vapor pressure. Fully passive temperature-limiting approaches assume worst-case conditions—effectively reducing the effectiveness of the EGR cooler in order to avoid coolant overheating at extreme conditions. Alternatively, in U.S. Pat. No. 6,367,256, a portion of an exhaust flow is by-passed around an EGR cooler under conditions of low coolant flow and high EGR flow. To avoid coolant overheating, the heat-exchange process is dialed down. By providing a reduced rate of exhaust cooling, however, this approach may fail to enable the full range of benefits of cooled EGR.
The inventor herein has recognized these issues and has devised a series of approaches to address them. Therefore, one embodiment of this disclosure provides a method for operating an engine system having a cylinder, an exhaust turbine, and an intake-air compressor. In this method the cylinder is charged with exhaust from upstream of the turbine (internal or high-pressure EGR) at a first rate. The cylinder is charged with exhaust from downstream of the turbine (low-pressure EGR) at a second rate. The method further comprises increasing the second rate relative to the first rate in response to a coolant-overheating condition. In this manner, more of the exhaust heat is discharged directly to the ambient air during the coolant-overheating condition, without passing through the coolant. Such an approach may extend the benefits of cooled EGR over a larger portion of the engine map, while still providing the desired overall level of exhaust residuals.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted herein.